As shown in FIG. 1, electrochemical batteries are generally modular in structure. A base element is constituted by an individual storage cell A, made up of a single electrochemical cell. A plurality of storage cells connected in parallel forms a storage cell unit CA; such a unit delivers a voltage equal to the voltage of a single storage cell, but delivers greater current and storage capacity. In order to raise the voltage level delivered by the battery, a plurality of storage cell units are connected in series, forming a so-called “module” M. A plurality of modules may in turn be connected in series to form a so-called “stack” S. A complete battery BATT is made up of a plurality of stacks connected in parallel.
The system of the invention seeks in particular to enable voltage to be balanced between the various elements (cell units, modules) of a battery, that are connected mutually in series.
The problem of voltage balancing is illustrated by FIG. 2A which shows a battery made up of an association of four storage cell units CA1, CA2, CA3, and CA4, which battery is connected to a current generator in order to be charged. Ideally, all four units should be charged to the same voltage of 4 volts (V) in order to provide an overall voltage of 16 V at the terminals of the battery. In reality, there exist dispersion phenomena associated with conditions of fabrication, utilization, and aging of the elements, which cause some of them to charge or discharge more quickly than others, or to do so at different voltage levels. Thus, in FIG. 2A, the elements CA1 and CA4 are charged to a voltage of less than the nominal value 4 V, while the element CA2 is charged to a value that is significantly greater (4.3 V) and that might damage it. Conversely, the element CA4 is charged to a voltage of only 3.8 V; this element thus runs the risk after prolonged use of being in a deep discharge state, which is just as damaging, and which might not be detected by measuring only the voltage across the terminals of the series association. These problems are particularly acute with lithium batteries which are very sensitive to undercharging and to overcharging.
A similar problem occurs when associating photovoltaic cells in series, as is needed to raise the voltage level delivered by a single cell. If one of the cells in the association is faulty, or is merely exposed to light flux that is less intense than the others (because its surface is dirty, or because it is in the shade), a negative potential difference may appear across its terminals, thereby greatly reducing the overall power level generated by the association.
FIG. 2B shows such a series association of photovoltaic cells PV1, . . . , PVN represented by reverse-biased diodes. A maximum power point tracker (MPPT) connected in series with the cells determines the magnitude of the current flowing through the series association so as to maximize the power generated by the photovoltaic effect. In FIG. 2C, curve CIV1 shows the current (I)/voltage (V) characteristic of the photovoltaic cells when exposed to the same light flux; the curve CIV2 the characteristic of a cell that, starting from an instant T, is exposed to a lesser light flux, e.g. because of dirt.
For t<T, when all of the cells are illuminated in the same manner and therefore follow the same characteristic CIV1, the MPPT module imposes a current IOPTI through the series association, and that leads to a potential difference VOPTI across the terminals of each cell, such that:POPTI=n·VOPTI·IOPTI=n·max(V·I)
Starting from the instant t=T, one of the cells, PVi, receives less light flux, and its characteristic becomes that of the curve CIV2.
If the current flowing through the series association remains equal to IOPTI, the potential difference at the terminals of the shaded or dirty cell PVi becomes negative and equal to −VB, (avalanche breakdown voltage). The loss of power is thus equal to:ΔP1=−IOPTI(VOPTI+VB)
The MPPT module may react to this situation by reducing the current to the level I′=IOPTI−ΔI, such that the cell PVi produces energy once more. Nevertheless, the total power is reduced toP′=I′·[(n−1)·V2+V′]with a loss of powerΔP2=P′−POPTI 
V2 being the voltage across the terminals of the cells PVj (j≠1) for I=I′ and V′ being the voltage across the terminals of the cell PVi for I=I′.
In any event, it is important to observe that merely reducing the illumination of a single photovoltaic cell leads to a significant reduction in the power generated by the series association.
In order to mitigate the drawbacks of series associations of elements for generating and/or storing electrical energy—where electrochemical storage cells and photovoltaic cells are merely non-limiting examples of such elements—it is necessary to provide balancing systems.
The state of the art includes several voltage balancing systems for elements for electrochemically storing electrical energy.
The most common balancing systems are of the passive or dissipative type. For example, while charging, those systems act continuously or periodically to measure the potential difference across the terminals of each of the series-connected elements, and they divert to a dissipater resistor the current that can no longer be absorbed by the elements of smaller capacity. It can be understood that such systems lead to losses of energy that are difficult to accept; in the event of wide dispersion in the characteristics of the various electrochemical elements, the size of the heat dissipaters can become prohibitive. Discharging of the battery must be stopped when the lowest capacity elements have reached their low acceptable voltage limit; this means that the storage capacity of the battery is limited by the storage capacity of its worst elements.
There also exist active balancing systems that redistribute current within the battery instead of dissipating it. Thus, while charging, such systems divert the current that can no longer be absorbed by the “weaker” elements to the “stronger” elements, having storage capacity that is not yet used up. While discharging, they take additional current from the “stronger” elements in order to compensate for the lack of current coming from the “weaker” elements. The main drawbacks of such systems are their complexity and their high cost.
The article by N. Kutkut and D. Divan “Dynamic equalization techniques for series battery stacks”, 18th International Telecommunication Energy Conference, 1996 (INTELEC'96), pp. 514-521, describes several active balancing systems.
The simplest of those systems establishes a bypass path for each element, each of said paths including a switch that is normally open. When an element reaches its maximum charge level, the switch closes, thereby diverting the charging current to an energy storage inductance. After a certain length of time, the switch opens again, and the energy stored in the inductance is delivered to the battery element located immediately downstream in the series association. The cycle then restarts. The drawback of that system is that it allows energy to be transferred in one direction only, from “upstream” elements (close to the cathode of the battery, thus situated at a higher electric potential) towards “downstream” elements (close to the anode, and thus situated at a lower electric potential). In order to achieve transfer in both directions, it is necessary to provide a structure that is more complex, forming a direct current/direct current (DC/DC) voltage converter of the half-bridge type with an inductive load connected between each pair of adjacent battery elements in the series association.
Document U.S. Pat. No. 6,150,795 describes an active both-way balancing system in which energy can be transferred between adjacent battery elements via respective magnetic couplers. As in the above-described situations, energy transfer takes place only from neighbor to neighbor.
Other systems of a centralized type perform overall balancing of battery elements by means of a multi-winding magnetic coupler to which all of the elements are connected via respective switching circuits. An example of such a system is described by document U.S. Pat. No. 6,873,134—see in particular its FIG. 11, which is considered as constituting the closest prior art. Such a system is shown in simplified manner in FIG. 3. References CA1, CA2, . . . , CAi, . . . represent different storage cell units connected in series. The reference NM labels a magnetic core that is common to all of the storage cell units and that acts as a coupler. A plurality of windings W1, W2, . . . , Wi, . . . associated with the respective storage cell units are made on the core NM. More precisely, these windings are double windings, each having two end contacts P1 and P3 with a midpoint P2. The midpoint P2 of each winding is connected to the positive terminal (cathode) of the corresponding storage cell unit; the two end contacts P1 and P3 are connected to its negative terminal (anode) via respective transistors T1, T2 that form a half-bridge inverter. The transistors T1 of the various units are controlled synchronously; the same applies in complementary manner to the transistors T2. It can be understood that the magnetic flux induced in the magnetic core NM changes sign depending on whether it is the transistors T1 that are conductive while the transistors T2 are non-conductive, or vice versa. By activating the switching T1 and T2 in alternation, so-called “natural” balancing of the charging voltage of the battery elements is obtained: elements with greater charge deliver energy to elements with smaller charge via the magnetic core NM.
The main drawback of the balancing system of FIG. 3 lies in its bulk, in particular because a double winding with a midpoint is used for each battery element (storage cell, storage cell unit, or module), to be balanced. Another drawback is that the system is not capable of handling the failure of a storage cell unit.
The problem of balancing or compensation in series associations of photovoltaic cells is known in particular from the article by T. Shimizu et al. “Generation control circuit for photovoltaic modules”, IEEE Transactions on Power Electronics, Vol. 16, No. 3, May 2001. That article proposes a first circuit based on using a magnetic coupler and performing balancing of centralized type. That circuit is of relatively large size. The article also discloses a second balancing circuit based on a multi-stage chopper circuit for which control is relatively complex.
The article by T. Mishima and T. Ohnishi “Power compensation system for partially shaded PV array using electric double layer capacitors”, 28th Annual Conference of the IEEE Industrial Electronics Society (IECON 02), 5-8 Nov. 2002, Vol. 4, pp. 3262-3267, discloses an alternative balancing circuit for series associations of photovoltaic cells making use of capacitive storage of electrical energy. That circuit is both complex to control and also bulky, since it relies on using several banks of relatively high capacitance capacitors.